Dealing with navbar is one of the most complicated thing for me in html/css (seriously) and today I saw this project and wanted to make a navbar similar to this one, but I just can’t figure out how to do it.

Why don’t you show us some code you are working on? Are you trying to build the menu with HTML and CSS, or are you trying to actually make the menu work? If you are trying to get the menu to work, you will need JavaScript as HTML and CSS will not suffice.